Method of forming electric insulating films oriented silicon steel

ABSTRACT

A method for producing a glassy film having an excellent insulating property on an oriented cold-rolled steel sheet by coating the surface of said silicon steel sheet with a mixture of a Ti compound and an Mg compound and possibly with the addition of a Mn compound thereto and then subjecting the coated silicon steel sheet to a heat-treatment.

United States Patent [1 1 Yamamoto et al.

[4 Feb. 25, 1975 Filed: Aug. 16, 1973 Appl. No.: 388,790

Related U.S. Application Data Continuation of Ser. No. 207,524, Dec. 13,1971, abandoned, which is a division of Ser. No. 781,963, Dec. 6, 1968,Pat. No. 3,627,524.

Foreign Application Priority Data Dec. 12, 1967 Japan 42-79243 U.S. Cl148/113,1l7/135.1, 117/221,

148/31.57 Int. Cl. Holf 1/04 Field of Search 148/113, 112,111, 31.55;

[56] References Cited UNITED STATES PATENTS 2,533,351 12/1950 Carpenter148/113 3,151,000 9/1964 Schmidt et a1 148/113 3,203,839 8/1965Takahashi et a1... 148/113 3,389,006 6/1968 Kohler 148/113 3,418,71012/1968 Seidel et a1. 148/113 3,627,594 12/1971 Yamamoto et a1. 148/1137/1972 Matsumoto 61: a1. 148/111 Primary ExaminerWalter P. SatterfieldAttorney, Agent, or Firm-Wenderoth, Lind & Ponack [5 7 ABSTRACT A methodfor producing a glassy film having an excellent insulating property onan oriented cold-rolled steel sheet by coating the surface of saidsilicon steel sheet with a mixture of a Ti compound and an Mg compoundand possibly with the addition of a Mn compound thereto and thensubjecting the coated silicon steel sheet to a heat-treatment.

6 Claims, No Drawings METHOD OF FORMING ELECTRIC INSULATING FILMSORIENTED SILICON STEEL This application is a continuation of Ser. No.207,524, filed Dec. 13, 1971 and now abandoned, which in turn is adivision of application Ser. No. 781,963, filed Dec. 6, 1968 now U.S.Pat. No. 3,627,524.

This invention relates to a method for improving the brittleness of anoriented cold-rolled silicon steel sheet and at the same time forming anelectrical insulating film having an excellent electric insulation,adhesion, bendability, space factor and heat resistance on the surfaceof the oriented cold-rolled silicon steel sheet. The

term oriented cold-rolled silicon steel sheet used in the presentinvention means a cold rolled silicon steel sheet consisting of 2 to 3.5percent by weight Si, the rest being Fe and unavoidable impurities, andhaving a highly oriented crystal grain structure after being finallyannealed. Further, it contains .in addition one or more of the materialsAl (0.01 to 0.09 percent by weight so]. Al), sulfide, nitride of Mn andV and selenium compound, in small amounts as an inhibitor in order toeffect a highly oriented secondary recrystallization during the finalannealing. The expression oriented" means that .a .great part of crystalgrains have (110) [001] crystal orientation, which is called cube onedge or (100) [001] crystal orientation, which is called cube on face.

Oriented cold-rolled silicon steel sheet is used for laminated ironcores or wound iron cores. Such individual silicon steel sheetsarecoated with .electrical insulating films so as to be electricallyinsulated from each other.

Such electrical insulating film is required to be so tightly adhered tothe sheet that-it will not be peeled off during working and to have ahigh space factor and excellent resistance to heat. On the otherhand,such insulating film should have no bad influence on the magneticproperty ofthe steel sheet. The silicon steel sheet is generallysubjected to a final annealing in which it is treated at a hightemperature to about 1,200C for a long time.

Such final annealing is carried out on steel sheets which are laminatedor wound in coils. In such case, in order to prevent the steel platesfrom sticking to each other due to heat, an annealing separator is used.

For such annealing separator there is utilized a substance which acts toprevent sticking of the steel sheet and at the same time forms a glassyelectrical insulating film by reacting with an oxide on the surface ofthe steel sheet at the annealing temperature.

Substances used for such separators and methods for producing glass-likeelectrical insulating film are known from U.S. 'Pat. Nos. 2,354,123,2,385,332, 2,492,682 and 2,533,351.

These known separators are provided not only for separating action, butalso to form electrical insulating films. Therefore, various filmcharacteristic improvin steps can be taken.

For example, there is known a process wherein, during a decarburizingannealing, when Si is selectively oxidized to form SiO on the steelsheet surface, the steel sheet surface is coated with a suspensionconsisting of MgO, Mg(OH) and H and is dried and the steel sheet is thenfinal-annealed at a high temperature in a reducing atmosphere, and theMgO will act as annealing separators to prevent the steel sheets fromsticking to each other and at the same time a part of the materials willreact with the above-mentioned SiO to form a glassy electricalinsulating film.

However, 'MgO is used as the substance to act as a separator and as aglassy film forming agent. When it is to be applied to a steel sheet, itis generally mixed with water so that itis in the form of a liquidsuspension or slurry.

Therefore, there is a disadvantage that MgO acts with water to becomeMg(Ol-l) which discharges water of hydration while being heated andoxidizes and embrittles the steel sheet.

Such embrittlement causes considerable trouble during the operation ofcutting, perforating or winding steel sheets.

As a result of various research made with a view to eliminating theabove-mentioned disadvantage and further to reduce the brittleness ofsteel sheets, the inv.en tors have found that, when a Ti compound suchas an oxide or hydroxide of Ti and a Mn compound such as an oxide orhydroxide of Mn is mixed with a Mg compound such as an oxide orhydroxide of Mg, an excellent stable and strongly adherentglassyelectrical insulating film can be formed on an orientedcold-rolled silicon steel sheet and at the same time the brittleness ofsaid steel sheet can be reduced.

An object of the present invention :is to provide an insulating filmforming process for stably forming a film having an excellent electricinsulation property and heat resistance and which is strongly adherentand at the same time reducingthe brittleness of the oriented cold-rolledsilicon steel sheet itself.

It is known that, in order to form an adhesive insulating film,'SiO 'canbe formed on the surface of a silicon steel sheet'by selectiveoxidization so as to be a silicious base for the later formation of aglassy film. Such a layer of the silicious base is formed generally atthe time of decarburizing annealing. However, in general, the layercomposition is so sensitive to the temperature, time, atmosphere and dewpoint of the atmosphere that it is difficult to obtain a high qualityglassy film in a stable manner with known substances capable of servingboth as a separator to prevent sticking and as a glassy film formingagent with the result that a film which is poorly adherent is oftenproduced.

Particularly, when carrying out a treatment for forming .theabove-mentioned silicious base layer, when Al in an amount of 0.01 to0.09 percent by weight so]. Al

' is contained in the silicon steel sheet as an inhibitor, the

Al will also be partly oxidized 'to produce A1 0 which conjointly withthe treating conditions will have a great influence on the formation ofa uniform glassy "film such as impairing the stability of the glassyfilm and making it more difficult to form a film with excellent electricinsulation characteristics, appearance and .adhesion.

Further, the water contained in the abovementioned substance will bedischarged in the final annealing, which will improperly oxidize thesteel sheet and prevent the formation of a high quality glassy film at atemperature particularly above 1,000C.

The present invention comprises heat-treating for a short time anoriented silicon steel sheet coldrolled to any desired dimensions, forexample, in an atmosphere containing wet hydrogen gas so that a surfacelayer containing SiO from the selective oxidation of Si in the drogen sothat a glassy insulating film will be formed According to the method ofthe present invention, a uniform glassy film which is strongly adherentis stably obtained without being influenced by the heat-treatingconditions of the atmosphere and the like. Further, the

brittleness of the steel sheet itself is greatly reduced.

The method of the present invention shall be explained more particularlyin the following.

As mentioned above, the present invention is' to be applied to orientedcold-rolled silicon steel sheets. Such an oriented cold-rolled siliconsteel sheet is annealed continuously for a short time under a condition,for example, in a wet reducing atmosphere which prevents the oxidizationof iron as much as possible, but allows the oxidization of silicon inthe steel so that SiO will be formed on the surface of the steel sheet.

The heat-treatment for the formation of the abovementioned SiO- iscarried out under a condition which will prevent the oxidization ofiron. However, a slight oxidization ofiron is not always detrimental.But excess oxidization will reduce the quality of the glassy film andtherefore must be avoided.

For the heat-treatment for forming 'SiO by the selective oxidization onor near the surface of an oriented cold-rolled silicon steel sheet, areducing atmosphere of hydrogen only or dissociated ammonia is generallyused, the dew point of which is 55 to 70C and the temperature of whichis 700 to 900C. The longer the treating time, the thicker the oxidizedlayer and the greater the advantage in the formation of a glassyinsulating film. But, the holding time at the abovementioned temperatureis at least one minute or more than one minute. Needless to say, evenwhen, instead of the short time treatment, a box-annealing is .carriedout with open coils to form a layer containing Si on the surface of theoriented cold-rolled silicon steel sheet, the same favorable result willbe obtained. But, in such case, the dew point is preferably below 55C.

During the above-mentioned heat-treatment for the formation of SiO thedecarburization of the oriented cold-rolled silicon steel sheet can besimultaneously carried out.

The oriented cold-rolled silicon steel sheet surface on which a layercontaining SiO has been formed is further coated with a substance havingas main components one or more Mg compounds such as MgO and Mg(OH) andone or more such Ti compounds, such as TiO TiO .H O, TiO.(OH) andTi(Ol-l) It is most economical to use this substance as a glassyfilmforming agent and also as a material which prevents sticking of thesteel sheets during the final annealing.

The above-mentioned substance is formed by mixing 0.5 to 40 parts byweight of a Ti compound with 100 parts by weight of an Mg compound.

When less than 0.5 part of TiO is used, there will be no effect of theTi compound and the object of the present invention cannot be attained.On the other hand, when more than 40 parts of the Ti compound are used,no high quality glassy film will be obtained.

Therefore, 0.5 to 40 parts of the Ti compound are used to make the filmagent.

Further, when the amount of the Ti compound is larger than 40 parts, thesubstance after being dried will be likely to peel off the sheet duringhandling, the formation of the insulating film will be difficult, asurface having a metallic luster will be exposed and no uniform glassyfilm will be produced, and even if a phosphatetreatment is thereafterused, no highly adherent film will be obtained.

Further, the Ti compound in the substance has an excellent effect inthat it reduces the brittleness of the steel sheet. When 0.5 part of theTi compound is mixed with 100 parts of the Mg compound, the effect ofreducing the brittleness of the steel sheet will appear. When the amountis above the upper limit, there will be no substantial brittlenessreducing effect. It is only when 5 to 20 parts of the Ti compound areused that the brittleness will be most reduced.

MgO, which is-one of the components of the substance to be used in thepresent invention, may be either a high-temperature baked product or alowtemperature baked product. Howver, the lowtemperature baked MgO ismore effective for attaining the object of the present invnetion. Itsgranularity can be as fine as about 325 meshes to obtain a favorableresult.

TiO can be either the rutile type or the anatase type or can be obtainedby either low-temperaturedehydrating or high-temperature-baking ofmetatitanic acid. lts granularity can be fine as about 325 meshes, thesame as the above-mentioned MgO to obtain a favorable result.

The higher the purity of such film substance, the better the result.However, even with a reagent of about the first grade or a commercialindustrial product, the object of the present invention can be attained.

The substance of the above-mentioned composition is preferably mixedwith water so as to form a slurry and the slurry is applied to thesurface of an oriented cold-rolled silicon steel on which a layer of SiOhas been formed by selective oxidization. The amount applied ispreferably such that the amount of the substance which remains afterbeing dried will be '2 to 10 g./m When the amount is 4 to 8 g./m thebest result will be obtained.

After the above-mentioned coating, the sheet is dried and is then heatedat a high temperature in a reducing atmosphere so that a glassyinsulating film will be formed. It is most economical to carry out thisglassy insulating film forming treatment at the time of the finalannealing of the oriented cold-rolled silicon steel sheet.

That is to say, the oriented cold-rolled silicon steel sheet coated withthe above-mentioned substance and then dried is wound up in the form ofa coil or sheets cut to proper dimensions are stacked and finalannealedduring which it is held at a high temperature above l,100C, such as, forexample, 1,200C for more than 5 hours in an annealing furnace in areducing atmosphere such as, for example, of pure hydrogen.

In such case, SiO present on the surface of the oriented cold-rolledsilicon steel will react with the substance to form a blackish lustrous,compact and rustproof, acidproof high quality glassy insulating filmhaving a composition in the SiO -MgO-TiO system.

In the above-mentioned final annealing, the lower the dew point of theatmosphere as the temperature rises, the easier the formation of a highquality film. The substance applied to the surface of the orientedcold-rolled silicon steel sheet is also an annealing separator in thefinal annealing. That is to say, only a part of the applied substancereacts with Si0 formed on the surface of the steel sheet so as to form aglassy film and substantially the greater part of it remains as anannealing separator on the glassy film formed on the steel sheetsurface. Therefore, in order to make the steel sheet a product, it isnecessary to remove the powder by such a means as brushing or pickling.

By the above-mentioned treatment of the present invention, a highquality glassy insulating film is formed on the surface of an orientedcold-rolled silicon steel sheet and, at the same time, as anotherfeature of the present invention, the brittleness of the orientedcoldrolled silicon steel sheet itself is reduced. This is presumed to bebecause the mixture of MgO-TiO produced during the treatment reacts withSiO on the surface of the oriented cold-rolled silicon steel sheet toform a compact high quality glassy film, and therefore the orientedcold-rolled silicon steel sheet is not influenced by the annealingatmosphere and the Ti compound acts to directly purify the impurities inthe oriented cold-rolled silicon steel sheet.

Further, in the case of selectively oxidizing an Al containing orientedcold-rolled silicon steel sheet so that SiO will be formed on thesurface, even when Al is also oxidized to produce A1 0 theabove-mentioned substance will be effective. However, the presence ofthis Al O will make it difficult to obtain a uniform glassy film and,with the slightest fluctuation of the treating conditions,'theformationof the glassy film will be influenced so as to be unstable and a filmwhich is poorly adherent will be likely to be produced:

Therefore, by carrying various experiments to solve the above-mentionedproblems, the inventors have discovered that, by using the sametreatment as is described above by adding to the substance which is themixture of a Mg compound and a Ti compount a Mn compound in the form ofan oxide or a hydroxide, such as MnO Mn O- Mn (OH) or Mn0.0H or MnCOwhich becomes MnO when heated, a uniform lustrous glassy film which isstrongly adherent can be stably formed on the surface of an orientedcold-rolled silicon steel sheet.

In the substance consisting of the Mg compound, Ti compound and Mncompound, the Mn compound is considered to act to suppress the influenceof the A1 0 formed on the steel sheet surface. That is to say, whenheated, the Mn compound will become MnO, will be further reduced by theatmosphere and will enter the steel. The reaction in such case isconsidered to effectively act on the formation of a film. Further, theoxygen produced by the decomposition of the Mn compound in the reducingatmosphere below 1,000C is also considered to effectively act. MnO isexcellent in that it is a compound which discharges oxygen and isreadily available. Its granularity should be about 325 meshes to obtainfavorable results.

In an experiment, when an oriented cold-rolled silicon steel sheetproduced from a hot-rolled silicon steel sheet containing 0.030 percentsol. Al was coated with a substance within the scope of the presentinvention, was then heat-treated and was taken out and the substance wasanalyzed, the content of Al O was 1.1 to 21.1 percent.

The substance effective for application to the abovementionedAl-containing oriented cold-rolled silicon steel sheet is formed bymixing 0.5 to parts, preferably 2 to 40 parts, of a Ti compound and 0.5to 50 parts, preferably 2 to 25 parts, of a Mn compound with I00 partsof a Mg compound.

In the above-mentioned composition, when the Ti compound is below theabove-mentioned range. no effect of the compound will be obtained and,on the other hand, when it exceeds 80 parts, the substance after beingdried will be likely to be peeled off by the slightest contact or shockand the formation of the insulating film will become difficult.

Further, when the Mn compound is below the abovementioned range, it willnot be effective. On the other hand, when it is used in excess of 50parts, the amount of oxygen discharged during the heat-treatment will beso large that the atmosphere will become oxidative and will have a badinfluence on the formation of the glassy insulating film;

The substance of the above .composition is not only effective forapplication particularly to an Alcontaining oriented cold-rolled siliconsteel sheet, but also can be applied to such oriented silicon steelsheet containing no Al as is described above.

The substance of the present invention has been described in detailabove. Furthere, a comparison of the differences in characteristicsobtained, when known substances using MgO and the substances of thepresent invention are used, but the other conditions are exactly thesame, will be described with reference to examples in the following.

EXAMPLE 1 An oriented cold-rolled steel sheet having 3.25% Si andreduced to a final thickness of 0.35 mm was annealed for a short time of5 minutes at a temperature of 800C in a wet dissociated ammoniaatmosphere having a dew point of 6lC so that SiO was selectively formedon the steel sheet surface, was coated with a substance at each of themixing rates in Table 1 so that the amount coated on the sheet was 6gJm". and was final-annealed for 2.0 hours at a temperature of l,200C ina dry hydrogen atmosphere. The characteristics of the thus formed glassyinsulating films are shown in Table 2. The films made by the method ofthe present invention are more compact and much more strongly adherentthan by the known processes.

Further, in Table 3 there are shown the results of bending tests by theASTM method to show the effects of reducing the brittleness of the steelsheets after the formation of glassy films. It is clear that thebrittleness is greatly reduced in the films made by the method of thepresent invention.

TABLE I COMPOSITIONS OF USED FlLM AGENTS present invention TABLEI-Continued COMPOSITIONS OF USED FILM AGENTS TABLE 4 COMPOSITIONS OFSUBSTANCES Composi- Composition tion Sample MgO Ti0 Water Sample MgO TiOMn0 Water No. No.

3 100 g. 25 I lit. do. 1 100 I 4 100 40 1 m dfi. 8 2? g do. $31133:

Control Substance (A) 5 100 g. 100 I In. ($102 was not 3 do. 20 0 do. ofthe present formed) invention 4 do. 20 6 do. Substance (B) of thepresent 5 d 20 3 d inventioni o. I 0. 0. TABLE 2 6 do. 27 7 do. do. 7do. 30 14 do. do. 8 do. 36 7 do. do. GLASSY FILM CHARACTERISTICS 9 do 4015 do. do Proper- Layer Resistance Adhesion After Phosphate- 10 6 $9 3no! ties Under 35 1 g./cm Coating as Bent by 180 m Sample by ASTM method2 to a Diameter of 20 mm.

l 4.30!) cm.lsheet partly peeled off TABLE 5 2 1 1.5 do. not peeled off3 19.4 l' l 4 928 3g g g g g gf g g GLASSY FILM CHARACTERISTICS 5 0.20do. 1 almost peeled off charac- 4 teristics Layer resistance Adhesionafter phosphate- $13111!212?KIRZZ2Q5 1$283831.itfifllfis'lfi fiZ1 13?I'IJFYKJZSPZ Sample (under 35 -l coming as hem y coating on the film was3 g./m"'. N0. by ASTM method 2) IO 8 dlameter 0f 20 mm.

1 0.49 .Qcm ./sheet peeled off 2 0.91 do. partl peeled off TABLE 3 36.15 do. a litt e peeled off 4 10.5 do. do. BRI'ITLENESS OF STEEL SHEETSON 5 0 Pee'ed WHICH GLASSY FILMS WERE FORMED 6 4 peeled 7 24.3 do.lIttle peeled off Charac- Bending frequency (by ASTM method) 5; g8 g8teris- Less than l l to 2O 2l to 30 more than 10 0:13 m peeled fftIcs 10tImes times times 31 tImes Sample l 20 2 6 I2 EXAMPLE 3 2 0 7 16 17 3 224 1| 2 40 An orIented cold-rolled sIlIcon steel havIng 0.3 perg 1, g 16cent total Al and 2.85 percent Si was rolled to a sheet 0.15 mm thick,was continuously annealed for a short time of 4 minutes at a temperatureof 850C in a wet EXAMPLE 2 decomposed atmosphere having a dew point of65C so that SiO was selectively formed on the surface, was An orientedcold-rolled steel sheet having 0.32 per-- coated on the surface with asubstance at each of the cent total A1 and 2.9 percent Si and reduced toa final below-mentioned mixing rates so that the amount of thickness of0.30 mm. was continuously annealed for a the substance after dryingwould be 6 g./m was dried short time of 4 minutes at a' temperature of850C in a and was then final-annealed for 20 hours at a temperawetdissociated ammonia atmosphere having a dew ture of 1,200C in a dryhydrogen atmosphere so that point of C so that SiO was selectivelyformed on the a glassy film was'formed. As a result, excellentelectrisurface, was then coated on the surface with a subcal insulatingfilms as described below were obtained.

Layer Resistance Surface Appearance Composition After Removing AfterRemoving Powder Powder MgO TiO MnO In em .lsheet (under 35 kg./cm 100710.41 No glassy film was produced. 20g. 1g. 0.5g 3.95 A uniform lustrousglassy film was produced.

stance at each of the mixing rates and with the compositions shown inTable 4 so that the amount of the substance remaining after being driedwas 5.5 g./m was dried and was then final-annealed for 15 hours at atemperature of 1,200C in a dry hydrogen atmosphere so that a glassy filmwas formed.

What is claimed is: l. A method for producing an electrical insulationfilm on an oriented cold-rolled silicon steel sheet containing 0.01 to0.09 wt percent sol. A1, consisting essentially of the steps of firstsubjecting the silicon steel sheet to a heat-treatment at a temperatureof 700 to 900C in a reducing atmosphere of either hydrogen or anatmosphere containing dissociated ammonia having a dew point of 55 to70C for at least one minute; applying to the surface of the steel sheetan aqueous slurry of a mixture of l) 2 to 40 parts by weight ofat leastone compound selected from the group consisting of an oxide of Ti and ahydroxide of Ti, and (2) 100 parts of by weight of a compound selectedfrom the groups consisting of an oxide of Mg and a hydroxide of Mg;drying the steel sheet coated with the slurry, and annealing the saidsteel sheet in a hydrogen-containing atmosphere at a temperature above1,100C.

2. The method claimed in claim 1 wherein the oriented cold-rolledsilicon steel sheet contains 2 to 3.5 wt. Si.

3. The method claimed in claim 1 wherein the heattreatment for formingSiO on the surface of the steel sheet comprises holding said steel sheetat a temperature of 700 to 900C for at least one minute in a reducingatmosphere of hydrogen only.

4. The method claimed in claim 1 wherein the heattreatment for formingSiO on the surface of the steel sheet comprises holding said steel sheetat a temperature of 700 to 900C for at least one minute in a reducingatmosphere containing dissociated ammonia and having a dew point of 55to C.

5. The method claimed in claim 1 wherein the aqueous slurry is preparedby mixing the mixture with water and is applied on the surface of theoriented silicon steel sheet on which SiO has been formed in an amountsuch that the amount of the deposited material left after it has beendried is 2 to 10 g/m*.

6. The method claimed in claim 1 wherein only one compound from eachgroup is mixed in said slurry.

1. A METHOD FOR PRODUCING AN ELECTRICAL INSULATION FILM ON AN ORIENTEDCOLD-ROLLED SILICON STEEL SHEET CONTAINING 0.01 TO 0.09 WT PERCENT SOL.AL, CONSISTING ESSENTIALLY OF THE STEPS OF FIRST SUBJECTING THE SILICONSTEEL SHEET TO A HEAT-TREATMENT AT A TEMPERATURE OF 700* TO 900*C IN AREDUCING ATMOSPHERE OF EITHER HYDROGEN OR AN ATMOSPHERE CONTAININGDISSOCIATED AMMONIA HAVING A DEW POINT OF 55* TO 70*C FOR AT LEAST ONEMINUTE; APPLYING TO THE SURFACE OF THE STEEL SHET AN AQUEOUS SLURRY OF AMIXTURE OF (1) 2 TO 40 PARTS BY WEIGHT OF AT LEAST ONE COMPOUND SELECTEDFROM THE GROUP CONSISTING OF AN OXIDE OF TI AND A HYDROXIDE OF TI, AND(2) 100 PARTS OF BY WEIGHT OF A COMPOUND SELECTED FROM THE GROUPSCONSISTING OF AN OXIDE OF MG AND A HYDROXIDE OF MG; DRYING THE STEELSHEET COATED WITH THE SLURRY, AND ANNEALING THE SAID STEEL SHEET IN AHYDROGENCONTAINING ATMOSPHERE AT A TEMPERATURE ABOVE, 1,100*C.
 2. Themethod claimed in claim 1 wherein the oriented cold-rolled silicon steelsheet contains 2 to 3.5 wt. % Si.
 3. The method claimed in claim 1wherein the heat-treatment for forming SiO2 on the surface of the steelsheet comprises holding said steel sheet at a temperature of 700* to900*C for at least one minute in a reducing atmosphere of hydrogen only.4. The method claimed in claim 1 wherein the heat-treatment for formingSiO2 on the surface of the steel sheet comprises holding said steelsheet at a temperature of 700* to 900*C for at least one minute in areducing atmosphere containing dissociated ammonia and having a dewpoint of 55* to 70*C.
 5. The method claimed in claim 1 wherein theaqueous slurry is prepared by mixing the mixture with water and isapplied on the surface of the oriented silicon steel sheet on which SiO2has been formed in an amount such that the amount of the depositedmaterial left after it has been dried is 2 to 10 g/m2.
 6. The methodclaimed in claim 1 wherein only one compound from each group is mixed insaid slurry.